For beginner piano: Concordi laetitia

The handy annotations section of Cantus selecti gives the date for this as 13th century, with the melody being from a prosa Orientis partibus and the more proximate source for the chant being Variae Preces (1892). Interestingly, this latter book starts the chant on “Do”, whereas more recent editions put it on “Fa” to be more correct for mode 6.